Telephone system



G. HECHT TELEPHONE SYSTEM Filed OCT... 25, 1940 Aug. 25, 1942.

/A/l/EA/TOR G. HECHT ,4T TOR/v5 y' Patented ug. 25, 1942 TELEPHONESYSTEM George Hecht, Astoria, N. Y., assigner to Bell TelephoneLaboratories, Incorporated, New York, N. Y., a corporation of New YorkApplication October 23, 1940, Serial No. 362,345

4 Claims.

This invention relates to telephone systems and particularly to thereceiving equipment of a transmission line over which alternatingcurrents, preferably within the voice frequency range, are transmittedfor effecting the operation of said equipment to control, in turn, theselective positioning of switching apparatus. More particularly, theinvention is in the nature of an improvement in the receiving equipmentof the telephone transmission line disclosed in Patent 2,269,022 issuedto G. Hecht and A. A. Lundstrom on Jan. 6, 1942. In said patent isdisclosed a telephone system of the type in which automatic switches arecontrolled by means responsive to alternating currents of differentfrequencies generated at a subscribers station and transmitted over aline terminating in a plurality of channel detectors each selectivelytuned to a different one of the signal frequencies and responsivethereto to operate a relay which, in

combination with a relay operated over another channel detectorresponsive to a current of another signal frequency, serves to functiona settable register, or sender, in which is locked-in a record of thedigit indicated by the two operated relays; that is, by the twofrequencies transmitted from the subscribers station.

In systems of this kind where the signal generating instrument is,usually a device comprising a plurality of keys or the like which areoperated to produce the different combination of frequencies to whichthe channel detectors are respectively tuned to respond, it very oftenhappens that, in depressing a digit key, two or more of them areaccidentally depressed together, in which event the amount of signalenergy produced may be greater than that available by the depression ofa single key (since only a small fraction of the physical energyexpended in depressing a key is utilized for conversion into equivalentelectrical energy) and the number of frequencies generated will be fouror more in stead of two, thus endangering the channel detectors tooperate falsely. However, the telephone transmission line disclosed inthe abovementioned patent to Hecht and Lundstrom employs a volumelimiting device which provides a ceiling for the maximum energy whichmay enter the channels. Since, in this case, the energy applied to thevolume limiting device is carried by four or more frequencies, theoutput energy for application to the channels will be the same but willbe carried by four or more frequencies instead of two. Under ordinarysignaling conditions, the quantity of output energy provided by theceiling of the volume limiter will be sufficient to operate two of thechannels if said energy is carried by two of the frequencies to one ofwhich each of the two channels is tuned to respond. But when the sameamount of energy is carried by more than two frequencies; the energy isdivided between more than two channels and that available for each ofthe channels may not be sufficient to operate them, in which eventl nodigit is recorded and the sender will function as though the subscriberhad omitted to transmit one of the digits of the wanted number.

In accordance with a feature of this invention, whenever the subscriberdepresses two or' more keys simultaneously and, as a result, thechannels will not operate for the reasons above stated, means areprovided whereby the sender is signaled to cancel whatever digits havealready been registered therein and to transmit reorder tone back to thesubscriber who will understand from this tone that he is to restore histelephone instrument, listen for key tone and thereafter depress thegenerating device keys all over again for the retransmission over theline of the complete number of the wanted subscriber.

This and other features of the invention will be more readilyascertained from the following detailed description of said invention,appended claims and attached drawing in which is shown schematicallycertain elements of the telephone system more completely disclosed inPatent 2,237,742 issued to A. A. Lundstrom on April 8, 1941, and inwhich certain other elements to which my invention more directlyrelates, are shown in detail.

Referring, now, to the drawing, A represents a subscribers station in anautomatic telephone system, said station being equipped with a device lsuitably arranged to produce one or more currents of differentfrequencies in the voice frequency range in response to the depressionof keys or the like that represent digits or other characters making upthe wanted designation, 2 represents a line extending from the stationinstrument to the central office whence, through one of a plurality ofselectable sender connectors I3, the line is extended to an idle senderwhich comprises coupling transformers 3, 4, 5, 6 and 1, amplifiers 8 andvolume limiter Il, the enabler circuit l0, relays 3| and 32 whichfunction from the enabler to signal the sender to transmit recorder toneaccording to the principles of the present invention, and the channeldetectors fI, f2, f3, f4 and f5 individually so designated in accordancewith the frequency to which each of the channels is respectively assumedadapted to respond, which channels include individually respondingrelays and the settable register relays which follow the operation ofthe channel relays when operated to signalize an incoming impulse. Theamplifiers 8 and 9 and the Volume limiter II are commercially availablecircuit units while the channel fI-f are completely disclosed in theabove mentioned patent to A. A. Lundstrom, and to which reference ismade for a more complete understanding thereof although it is to beunderstood that the invention is not limited to channels of thisdescrption.

In the operation of the invention, a call is initiated at station A bythe removal of the station instrument from the switchhook whereupon theline is then extended by any suitable means .to the'centraloiiice atwhich an available sender is Lthenconn'ected to said line through themediumo'f'a sender connector vI3A after which key .tonev fis.transmitted 'back to the calling station A, by circuits not shown, as asignal that the ,sending device Imay be operated totransmit the numberofthe wanted station, all in the manner: described in theabove-mentioned patent to A; A. Lundstrom.

When any. one of the 'ten keys of the sending device I'is opera-ted, twoout of the five dened'frequencies jfl-f5 are produced in the combinationrequired to-designate the digit represented by the ikey depressed andthe resulting current produced is transmitted over a circuit which maybe traced fromone-side of the sending device I, over the lower conductorof the looped pair 2, lower back contact of relay I2, next to the innercontacts of sender connector cut-in `relay I3, primary winding oftransformer 3inner contactsv of relay I3, top outer back contact of,relay I2, upper yconductor of looped pair 2- to the other :side of thestation sending device I. The current composed of the two frequencies isinduced in the secondary winding of Vtransformer 3 and fed into theinput side of amplilier 8 whence, after suitable amplification, itisfapplied from the output side to the input sides of transformers -4:and 5, respectively, whence through the transformer coupling it isfurther` applied, through transformer `4, to the input side of thevolume limiter II and, through transformer 5,;to the input side ofamplifier 9.

Considering that vportion ofthe signal energy which passes into thevolume limiter II, the energycauses the volume limiter to'operate 'andregulate the energy output in accordance with its adjustment which maybe suchas to' limit'the magnitude of output to that required for thesatis-factory opera-tion of the channel detectorsand'no more, regardlessof the magnitude of the input energy. The reason for doing this is, ofcourse, to prevent overloading any of the channel detectors'whic'h,should it occur in the event of `a heavyV input signal due, say, to avery short loop 2, would cause false operation of adjoining channeldetectors by currents present in the 'frequency spectrum generatedthereby and thus cause the registration of'a false number. Hence byVadjusting the volume limiter to a suitable ceiling of outputA energyabove-the operate level of the channel detectors, their false operationupon' heavy signal currents'is avoided.

`The-energy output from the volume limiter I I resistance pad I4 whenceit is applied through suitable filters BPF to the grids of the detectortubes in each of the channels fI-f. Since these tubes are normallyinactive, the channels that would be involved by the two frequencycomponents in the current transmitted through variable resistance padT4, do not operate.

Considering, now, that portion of the signal energy which is transmittedto the enabler circuit I Via transformer 6, it will be observed thatsaid enabler comprises a rectifier bridge I6 and four polarized relaysI1, I8, I9 and 22), each having a permanently closed polarizing circuitthrough its lower winding which causes the armature thereof to assumethe position indicated in the drawing. Further, between each two relays,vbeginning with relay I'I and ending with relay '3I, there is anintermediate condenser-resistance network which joins a contact of therelay on the left with the upper winding of the relay to the right.Considering relays I'I and I8, `for` instance, the network made up ofresistance-'23 and condenser 2l joins the right contact of relay II withthe upper winding of relay I8. Since the armature of relay I1 isconnected to positive battery 39, condenser 2| is isapplied, `throughtransformer'l, to the Variable 75 normally charged to the potential ofthis battery. Condenser 24 of the network between relays I8 and I9 onthe other hand, is normally at ground potential since it has-groundapplied to it through the left contact of relay I8. vContrariwise,condenser 29 of the network between relays I9 and 20 is charged to thepotential of the battery 3l) lthroughthe upper winding of relay 20 andresistance 2c since the short-circuiting ground path for condenser 29 isnormally open at the right contact of relay I8, which factleaves-condenser 29 exposed to the charging circuit that traces from thepositive pole of battery 30, through the upper winding of relay 20,resistance 28 and condenser 29 to ground. Similarly, condenser 3 isnormally discharged by the connection of ground to its top terminal viathe left contacts of relay 2), said ground also completing a normalcircuit through the top winding of relay 3I and resistance 34 tobattery. The current flowing through this winding counterbalances theeffect of the current flowing through .the always closed biasing circuitthrough the middle winding of said relay, thereb-y causing the armaturethereof to be disengaged from its right contact as shown in the drawing.

Assuming, therefore, that relays I'I, I3, I9, 2l) and 3| have theirrespective armatures in the positions shown, and that condensers 2l,2li, 29 and 33 are charged, discharged, charged and discharged,respectively, as indicated above, ground is normally disconnected fromconductor I5 at the right contact of relay 2l), which conductor is, inturn, connected to the cathodes of the detector tubes of the severalchannel detectors fI-f5, and causes them to be normally unresponsive tosignal energy incoming over the resistance pad Ill.

Now when a'portion of the incoming energy is applied to the rectierbridge I5, said energy is rectified andthe current thereof is applied tothe upper winding of relay Il. The combined effect of the current nowflowing through the upper winding of this relay and that always flowingthrough its lower winding is such as to cause its armature to swing fromthe right contact into engagement with its left contact. Since condenser2l is normally charged to the potential of battery 38 over the armatureand right contact of relay I1, the opening of these contacts immediatelycauses the condenser to discharge to ground through resistance 23 andthe upper Winding of relay I8 to prolong the flow of current normallyflowing through this winding of said relay. The direction of thedischarge current as well as the normal current through the upperwinding of relay I8 opposes that of the current normally flowing throughits lower winding, and since the discharge current is initially ofgreater intensity through the proper choice of the ohmic value ofresistance 23, the release of relay I8 is delayed and the armaturethereof is caused to break away from its left contact at a timesubsequent to the operation of relay I1 and to engage its right contact.

During the time that the armature of relay I8 is engaged with its rightcontact, and remembering that condenser 24 is kept at ground potentialwhile the armature of this relay is making with its left contact, then,as soon as the armature breaks with its left contact, condenser 24immediately begins to charge over a circuit traced through resistance 26and the upper winding of relay I9 to battery 30, to continue to maintainthe normal current over this path for a short time subsequent to theoperation of relay I8, whereupon the effect of the current through thelower winding of relay I9 continues to be neutralized for a short timeby this charging current so that the armature of relay I9 is caused tobreak with its left contact at a time subsequent to the operation ofrelay I8, that is, at the time when the charging current for condenser24 through the upper winding of relay I9 is no longer powerful enough toovercome that through the lower winding of the relay.

At the instant that the armature of relay I8 engages its right contactand before the armature of relay I9 leaves its left contact, a dischargepath is completed for condenser 29 which may be traced from ground,through condenser 29, re-, sistance 21, left contacts of relay I9 andright contacts of relay I8 to ground. The charge acf cumulated oncondenser 29 as a result of the charging current in the circuit throughthe upper winding of relay 20 is quickly drained off through the lowresistance 21 and a current flow is quickly established in the upperwinding of relay 28 for its rapid operation whereby ground is connectedto conductor I to sensitize the detectors so that, when the armature ofrelay I9 breaks with its left contact, condenser 29 is in an unchargedcondition and immediately starts recharging through the upper winding ofrelay 28 to prolong the current ow therethrough for a subsequent period.This charging current overcomes the effect of that normally flowingthrough the lower winding of relay 28 and causes the armature thereof toengage its front contact, thus applying ground to conductor I5 which, assaid before, is connected to the cathode of each of the detector tubesin the channel detectors fI--f5. The channel detectors are now maderesponsive and the two which are adapted to respond to the twofrequencies in the input current that flows into their respective inputside through resistance pad I4 operate, causing, in turn, the operationof their respective anode relays which, in their turn, cause theoperation of relays in the register to designate the digit or charactermarked by the two frequencies.

It is to be noted that relay I8 will make its right contact at a timesubsequent to the energizing of relay I1 by rectified current flowingthrough the rectifier bridge I6. This time is, of course, determined bythe discharge time of the condenser 2| through resistance 23 whichcauses the normal current through relay I8 to endure for a time (let ussay, for example, 20 milliseconds) after the operation of relay I1. Atthis time relay I8 releases and closes its right contact wherebycondenser 29 is quickly discharged through the low resistance'21 andestablishes a current through the upper Winding of relay 28 to cause itsimmediate operation. This, as eX- plained, places ground on conductor I5to sensitize the detectors at a time 20 milliseconds after the signalwas first applied. This delay is for the purpose of allowing theunwanted spectrum energy in adjoining channels to dissipate itself toavoid unwanted channel operation from this source. It is the purpose ofthe condenser 24 to maintain by its discharge current through the upperWinding of relay I9, the normal current flowing therethrough to holdrelay I9 operated on its left contact for a time sufficient to permitthe complete discharge of condenser 29 through resistance 21, fromcontacts of relay I9 and right contacts of relay I8 to ground. Relay I9then operates to break its left contact and thereafter relay 28 ismaintained on the charging current from battery 38, top winding of relay28, resistance 28, to condenser 29. When this discharge current hasdissipated itself, relay 28 will operate andbreak its back contact toremove ground from lead I5 regardless of how long thereafter that energyis delivered to relay I1 by the signal.

The length of time during which the armature of relay 28 engages itsright contact to cause the channel detectors to remain activateddepends, of course, upon the constants of condensers 24 and 29 andresistances 26 and 28 which may be chosen to provide any intervaldesired. However, when the condenser 29 is completely charged, thecurrent through the lower winding of relay 28 again becomes effective,the armature of the relay breaks away from its right contact and groundis removed from conductor I5 and, therefore, from the cathodes of thedetector element of the several detector channels which, in consequence,will no longer respond to any signal energy forthcoming over resistancepad I4 and thereby cause their associated anode relays to release.

The circuit conditions prevailing in the enabler during the presence ofsignal energy in bridge I6 and after the armature of relayv 28 has againmade its left contact, is that relay I1 will have its armature makingwith its left contact, relay I8 will have its armature making with itsleft contact, while relay I9 will have its armature on its left contactwhich will in no way affect the charge on condenser 29 or the conditionof the armature of relay 28 which is out of engagement with its backcontact. Hence it makes no difference how long signal energy prevailsupon the line subsequent to the armature of relay 28 breaking away fromits right Contact, said energy will be ineffective to operate thechannel detectors inasmuch as ground is disconnected from the cathodesof their several detector elements.

When relay 28 breaks its left contact to engage its right contact forthe purpose of applying ground to conductor I5 in order to sensitize thedetectors, the short circuit for condenser 33 is removed and a chargingcircuit for this condenser is immediately effective over resistance 34,top winding of relay 3| to battery. This charging current has the sameeiiect, for an interval of time, upon relay 3| as the current'previouslyowing through its upper winding directly from ground 'over the leftcontact of relay 2S, which is to say, that until the charging currentdecays to the point where it no longer is powerful enough to oppose theeffect of the current flowing through the middle Winding, the armatureof relay 3| remains disengaged from its right contact. The time.interval is, of course, determined by the capacity of condenser 33 andthe value oi-resistance 34 and both should be of such value as to delaythe pulling power of the current through the'middle winding of relay 3|until -sufcient time has been provided for the operation of any of thechannel relays in consequence of the signal current activating thecorrelated channel detectorsafter ground is applied to conductor I5.Under these circumstances the two channel detector relays, whenoperated, will close Va circuit through fthe lower winding of relay 3|over a path which may be traced, for instance, from battery through thelower winding of said relay, resistances and 22, over the innercontacts, respectively, of channel relay of detector channel fZ'anddetector channel JI to ground. The current that iiows through the lowerwinding of relay 3| is in thesame direction as that which flows throughthe top winding under normalconditions, and when condenser 33'is beingcharged it opposes the effects of the current flowing through the middlewinding of said relay so 4that if, when the charging current hasdecayed, the channel relays have operated and the circuit through thelower winding of relay 3| has been Y established, the current flowingtherethrough will oppose that flowing through the middle winding of saidrelay and the same will remain in its normal position; that is to say,with its armature disengaged from its right contact.

It may now be supposed that the subscriber, in the course of keying upthe desired number upon generating device I, accidentally depresses twokeys instead of the one designating the digit to be transmitted. In thiscase the quantity of alternating current generated will be more and willbe carried by at least three of the signal frequencies. Since the gainof the volume limiter II is set so that the output is never more thanmaximum permitted by the gain adjustment, the signal energy that seeksentry into detectors fI--JS will be divided between the three or morechannels involved. lThe portion of the energy thus received by eachchannel is not enough to cause the operation of its correspondingchannel relay so that no channel relay will operate under the conditionassumed. However, since by the time two of the channel relays aresupposed to operate the charging current for condenser 33 will havedecayed to the level at which it is powerless to oppose any longer thecurrent flowing through the middle winding of relay 3|, relay 3| willoperate on the current through said middle winding unless the operationcan be prevented by the closure of the circuit through the lower windingat one or more of the operated channel relays as before described. Butsince the channel relays cannot operate because of the insufficientsignal energy, the circuit through the lower winding is not closedwhereupon relay 3i operates and closes an obvious circuit for relay 32which, upon operating, locks through a set of contacts to olf-normalground. This relay may be used vto establish signaling circuits throughits contacts to appropriate parts of the sender for signaling the senderto transmit re-order tone to the calling station A and to cancelWhatever dig-its have already been registered in the sender. Since themanner and means byv which such-circuits are established form no part ofthe present invention, the same is not shown although by reference tothe sender disclosed in the abovementioned patent to A. A. Lundstrom, itwill be evident to anyone skilled in the art that the arrangement anddisposition of such circuits are relatively simple matters.

When the signal impulse is terminated (assuming the correct depressionof a key at device I), relay I'I will restore to its normal condition,re-

establishing quickly the charging circuit of condenser 2| and thecircuit through the top winding of relay I8 to cause it to make its leftcontact. Relay I8 operates and causes the quick establishment of thecircuit through the top winding of relay I9 to cause it to quickly makeits left -contact. In this operation, condenser 29 has not been charged.since ground via the contacts of relays |8 and I9 has not been removed.Therelease of the channel relays will open the circuit through the lowerwinding of relay 3|, but, since by this time the armature of relay 2liis already on the left contact, the circuit through the top winding ofrelay 3| is reestablished to oppose that through its middle winding andthus keep relay 3| normal. The enabler circuit is now ready for anothercycle of identical operations upon the reception of the next impulse.

While I have described my invention and the means for utilizing the samein connection with its specific application to a particularkind of atransmission line, it is to be understood that various otherapplications and embodiments thereof may be made by those skilled in theart without departing from the spirit of the invention as defined withinthe scope of the appended claims.

What is claimed is:

l. A register sender of a telephone system comprising a plurality ofdetector channels each adapted to respond to signal energy within adefinite frequency band, means responsive to incoming signal energy foractivating said detector channels to respond to said energy, and meansoperative in the event that the available signal energy is notsufficient to operate said detector channels subsequent to theiractivation.

2; In a telephone system, the combination with a telephone station andmeans located thereat to produce signal frequencies combined inpredetermined invariable numbers, of a sender connected to said linecomprising a plurality of channel detectors each adapted for response toa specio signal frequency of a combination generated at said station,means Vfor limiting the input signal energy for said channel detectorsto an amount sufficient to operate as many channels simultaneously asthere are signal frequencies in the invariable number in the combinationof fre-- quencies transmitted from said station, and means operative inthe event that said signal energy is carried by more than saidinvariable nurnber of signal frequencies and the energy carried by eachof the single frequencies is not suihcient to operate the associatedchannel.

3. In a telephone system the combination with a transmission linecomprising a plurality of channel detectors each adapted for response toa specific frequency of a plurality of frequencies present in a signalcurrent transmitted over said line but normally unresponsive thereto,means responsive to said signal current for activating said channeldetectors for response to their respective frequencies in said signalcurrent, and means for maintaining said channel detectors activated fora predetermined interval, said means including other means which areoperative in the event that none of said channels respond to said signalcurrent subsequent to activation.

4. In a telephone system, a transmission line terminating in a pluralityof channel detectors each adapted for response to a specic frequency ina plurality of frequencies present in a signal current transmitted oversaid line, and means responsive to said signal current for activatingsaid channel detectors for response to their respective frequencies andfor registering the failure of any of said channels to respond to saidsignal current, said means comprising a rectier coupled to said line, aplurality of relays cascaded from said rectifier of Which the first isresponsive to said signal current after rectification `by saidrectifier, the last of which operates to denote failure of any of thechannel detectors to operate, the next to the last of which appliesactivating potential to said channel detectors for a predeterminedinterval, and impedance networks intermediate each pair of saidplurality of relays except the first and last for controlling theiroperate time to make up said predetermined interval including the timerequired to operate l5 said last relay in the event that none of thechannels operate on said signal current.

GEORGE HECHT.

